1. Field of the Invention
This invention relates to the detection of electromagnetic radiation with a multiple quantum well (MQW) superlattice structure, and more particularly to the sensing of long wavelength infrared radiation (LWIR) with an MQW superlattice in which photoexcited charge carriers are transported through the superlattice under the influence of an electric field.
2. Description of the Related Art
MQW superlattice LWIR detectors made of heterojunction materials, such as GaAs/Ga.sub.x Al.sub.1-x As, provide good design flexibility for spectral response. The detection of LWIR with an MQW sensor has been reported in several publications, such as Levine et al., "Bound-to-Extended State Absorption GaAs Superlattice Transport Infrared Detectors", J. Applied Physics Letters, Vol. 64, No. 3, 1 August 1988, pages 1591-1593; Levine et al., "Broadband 8-12 .mu.m High-Sensitivity GaAs Quantum Well Infrared Photodetector", Applied Physics Letters, Vol. 54, No. 26, 26 June 1989, pages 2704-2706; Hasnain et al., "GaAs/AlGaAs Multiquantum Well Infrared Detector Arrays Using Etched Gratings", Applied Physics Letters, Vol. 54, No. 25, 19 June 1989, pages 2515-2517; Levine et al., "High-Detectivity D*=1.0.times.10.sup.10 cm.sqroot.Hz/W GaAs/AlGaAs Multiquantum Well .lambda.=8.3 .mu.m Infrared Detector", Applied Physics Letters, Vol. 53, No. 4, 25 July 1988, pages 296-298.
The principal of operation for an MQW superlattice IR detector is illustrated in FIG. 1. The basic device consists of a periodic heterostructure of GaAs quantum wells and AlGaAs barrier layers 4. The GaAs quantum well layers are doped with an n-type dopant, such as silicon, to provide electrons in the ground states of the wells for intersubband detection. The superlattice is sandwiched between a pair of heavily n-doped GaAs contact layers 6 and 8, with contact layer 6 functioning as an electron emitter and contact layer 8 as an electron collector during sensor operation. Ohmic contacts 10 and 12 on the opposed contact layers provide access to apply a bias voltage across the superlattice.
The thickness of each quantum well layer 2 is sufficiently small, generally about 20-60 Angstroms and most preferably about 40 Angstroms, that quantum effects are significant. The thickness of each barrier layer 4 is generally about 80-300 Angstroms, and most preferably about 140 Angstroms. The superlattice period is thus preferably about 180 Angstroms. It is generally preferred that the layers 2 are heavily doped n-type with a donor impurity such as Ge, S, Si, Sn, Te or Se. A particularly preferred dopant is Si at a concentration of about 1.times.10.sup.18 -5.times.10.sup.18 cm.sup.-3, and most preferably about 5.times.10.sup.18 cm.sup.-3. Lattice match and thermal coefficient considerations, impurity concentrations and fabrication techniques are known in the art.
Although a GaAs/AlGaAs superlattice is preferred, other materials may also be used. For example, it may be desirable to use materials such as InGaAs/InAlAs on InP, SiGe on Si, or HgCdTe. In general, superlattices fabricated from III-V, IV--IV and II-VI semiconductor materials are suitable. The MQW superlattice detectors are particularly suited for the detection of LWIR, but the sensors in general are applicable to the detection of radiation and other wavelength regimes, and no limitation to LWIR for the present invention is intended.
As illustrated in FIG. 2, the potential energy barrier height E.sub.b of the barrier layers 4 is about 160 meV above the potential energy barrier height E.sub.w of the quantum wells 2 for GaAs/AlGaAs; E.sub.f identifies the Fermi level. For LWIR with peak detection of about 12 microns, the energy gap between the bound state and the excited state for electrons in the quantum wells is about 100 meV, with the first electron excited state in the quantum wells lying above the conduction band edge of the barrier layers.
Incident infrared photons excite electrons from the quantized baseband 14 of the wells to extended excited states in a continuous conduction subband 16, which has an energy level greater than the conduction band floor for the barrier layers. The excited electrons are then accelerated towards the collector contact 8 by an electric field created by an externally applied bias voltage source V.sub.b (FIG. 1). Under normal sensor operating conditions, the bias voltage causes the mean-free path of electrons in the subband 16 to be sufficiently large for the electrons to travel under the applied field through the superlattice, producing a photocurrent that is measured as an indication of the magnitude of incident radiation. An ammeter 18 can be inserted in the circuit between ohmic layers 10 and 12 for this purpose.
The sensitivity of an MQW superlattice infrared detector can be severely limited by high levels of dark current. This current consists primarily of electrons which tunnel through the intervening barrier layers 4 between the ground states of adjacent quantum wells 2. The tunneling current can be reduced by increasing the widths of the barrier layers 4. However, any such increase in the barrier layer width reduces the device's radiation hardness, which is inversely related to its thickness.
An improvement upon the detector as described thus far is disclosed in co-pending U.S. patent application Ser. No. 07/457,613, filed Dec. 27, 1989 by Sato et al., "Dark Current-Free Multiquantum Well Superlattice Infrared Detector", and assigned to Hughes Aircraft Company, the assignee of the present invention now U.S. Pat. No. 5,077,593 issued Dec. 31, 1991. Under this approach the barrier layers 4 are kept thin, but a thicker (generally about 800-3000 Angstroms) tunneling current blocking layer 20 is provided at the end of the superlattice in the path of the tunneling electrons. The blocking layer 20, which is preferably formed from the same material as the barrier layers 4, eliminates most of the tunneling current component of the photodetector's dark current. This in turn allows the individual barrier layers 4 to be made thinner, thus enhancing the detector's quantum efficiency and increasing its radiation hardness.
Both the conventional MQW superlattice and the improvement offered by Ser. No. 07/457,613 require an external bias voltage to generate photocurrent. Without an applied bias voltage, photoexcited electrons move from their original quantum well regions to adjacent well regions in a random fashion, as indicated by arrows 17 in FIG. 2. As a result, the detector does not exhibit any net photocurrent. A power supply which adds to the cost, weight and bulk of the system is required for operation. This can be particularly undesirable for space applications, in which size and weight are at a premium. The addition of the tunneling current blocking layer 20, while solving the tunneling current problem, actually increases the bias voltage requirements. This is because the blocking layer is fairly high in resistance and carries most of the bias voltage applied across the overall device. The bias voltage must be substantially increased to produce a sufficient voltage differential and electric field across the superlattice. This in turn can lead to a reverse bias voltage breakdown across the blocking layer. Furthermore, it would be desirable to increase the quantum efficiency of both devices.
In an area that is not directly related to MQW superlattice detectors, the bases of heterojunction bipolar transistors (HBTs) have been provided with a graded dopant level or a graded Al content to induce internal fields in the base, and thus reduce the base-transit times of charge carriers. For example, see Kroemer, "Quasi-Electric and Quasi-Magnetic Fields in Nonuniform Semiconductors", RCA Review, Sep. 1957, pages 332-342. However, the principal charge carrier transport through the base of an HBT is quite different from that in an MQW superlattice.